The invention relates to methods and to apparatuses for active damping of vibration, for reducing and if possible cancelling the vibration of a mechanical member or of a medium.
The term "vibration" should be understood in a general sense and as defining phenomena that are periodic or random in nature and that may have a very wide variety of origins, e.g. vibration of a mechanical member or vibration of a fluid in the form of soundwaves propagating freely or in guided manner, within any frequency range.
A method is already known for active damping of vibration at at least one determined location subjected to a primary vibration field, whereby a vibratory wave is generated by adaptive filtering of a reference signal and for the purpose of interfering with the primary field at said location; the residual amplitude of vibration at said location is measured, and filtering is adapted automatically to tend to cancel the residual amplitude as measured in this way.
An apparatus is also known for active damping of vibration and enabling the above-defined method to be implemented: such apparatus includes at least one actuator capable of responding to adaptive filtering of a reference signal to generate a vibratory wave for interfering with the primary field at said location; at least one sensor providing an electrical signal representative of the residual amplitude of the vibration at said location; and means for adapting the coefficients of the adaptive filter as a function of the residual amplitude signal tending to cancel said residual amplitude.
By way of example, FIG. 1 shows a conventional structure of such an apparatus, intended, at a determined location, to reduce noise of any origin, but in which energy is concentrated at harmonic frequencies. The apparatus generally comprises a sensor 10 such as a microphone placed at the location to be protected, a generator 12 for generating a reference signal x, containing all of the harmonic frequencies at which the noise to be damped presents significant energy, a digital filter 14 receiving the reference signal, and a source 16 of vibratory waves for interfering with the primary noise field, and driven by the filter 14. Under such circumstances, the source 16 is generally consitituted by a loudspeaker that provides "counter-noise".
The filter is generally a digital filter using synchronous sampling and it consequently includes an input analog-to-digital Converter and an output digital-to-analog converter. It is adaptive responsive to an error signal e, having an amplitude and a phase representative of the residual noise picked up by the sensor 10. In general, for reasons of simplicity, adaptation is performed using the gradient algorithm. The coefficients of the filter 14 are continuously adjusted by performing correlation between the error signal e delivered by the sensor 10, and the signal x, obtained by processing the reference signal x in a loop filter 18 having a transfer function that is representative of the physical transfer function between the source 16 end the sensor 10 at each of the harmonic frequencies to be dealt with.
In many cases, the frequencies to be processed are multiples of a fundamental frequency 1/T.sub.o of the reference signal. This applies in particular to noise generated by a rotating shaft, where the fundamental frequency has a linear relationship with the rotation of the speed. The harmonic frequencies are then of the form n/T.sub.o where n is any positive integer. In practice, in most cases, it suffices to provide attenuation for a few values only n, corresponding to those frequencies at which the energy in the primary vibratory field to be damped is concentrated.
The speed with which the gradient algorithm converges depends:
on the autocorrelation matrices of the filtered reference signal x.sub.f ; and
on the convergence coefficient (iteration step) selected for the algorithm that adapts the coefficients of the digital filter.
When there is a large number of frequencies n/T.sub.o to be taken into account (i.e. when the number of energy peaks is high), then the autocorrelation matrices are such that convergence can become very slow.